(1) Field of the Invention
The present invention relates to an optical isolator device for use in an optical transmission system and, in particular, to such an optical isolator device having an improved optical isolation temperture characteristic.
(2) Description of the Prior Art
The optical isolator device is a device for permitting a polarized light beam to be transmitted therethrough in a forward direction along an optical axis but preventing a polarized light beam from passing therethrough in a reverse direction.
In an optical transmission system, the optical isolator is disposed between an oscillator (light source) and an end surface of the optical fiber transmission line. A light beam emitted from the oscillator passes through the optical isolator to the optical fiber transmission line. A portion of the light beam is reflected at the end surface of the optical fiber transmission line but is stopped by the optical isolator, so that the oscillator is protected from the reflected light beam and can effect the stabilized oscillation.
A conventional optical isolator device comprises a beam rotator for rotating the plane of polarization of an incident light beam by an angle of 45.degree. and two polarization selecting elements being disposed at opposite sides of the beam rotator on the optical axis for selecting and transmitting therethrough a polarized light beam having the plane of polarization in a special direction.
A Faraday rotator is usually used for the beam rotator. The Faraday rotator is a device using the Faraday effect which is one of magnetooptical effects and comprises a cylindrical magnet having a central axis and a magnetooptical element disposed on the central axis.
The polarization selecting element is, for example, a Rochon polarizing prism and has a transmission direction of the plane of polarization for allowing a light beam to pass therethrough only if the light beam has the plane of polarization coincident with the transmission direction. One of the two polarization selecting elements is disposed on an input side of the beam rotator for a light beam transmitted in a forward direction and is called an polarizer. The other one is disposed on an output side of the beam rotator for the forward light beam and is called an analyzer. The transmission direction of the analyzer is oriented at an angular position offset from that of the polarizer by 45.degree. in the rotating direction of the beam rotator.
In operation, the polarizer receives the forward light from the oscillator and selects and transmits a polarized component which is coincident with its transmission direction. The polarized component passes through the beam rotator with rotation by the angle of 45.degree. and emitted through the analyzer to the optical fiber transmission line. The emitted light beam is partially reflected at the end surface of the optical fiber transmission line and transmits through the analyzer in the reverse direction. The reverse light beam passes through the beam rotator and is rotated by the angle of 45.degree. at the beam rotator. Accordingly, the plane of polarization of the reverse light beam after passing through the beam rotator is offset by an angle of 90.degree. from the transmission direction of the polarizer, so that the reverse light beam is stopped by the polarizer. Therefore, the reflected light beam is prevented from being injected into the oscillator.
For a conventional optical isolator device, reference is made to No. JP-A- 57-17919 (Japanese patent application laid open with a publication No. 17919/82). For details of a known Faraday rotator, reference is made to U.S. Pat. No. 4,522,473 by Hibiya et al, assigned to Nippon Electric Co., Ltd.
However, the optical isolator device cannot completely block the reverse light beam from passing therethrough because the plane of polarization of the reverse light beam after passing through the beam rotator is not completely offset by 90.degree. from the transmission direction of the polarizer but has a deviation of an angle of several degrees due to a variation of the rotation angle of the beam rotator in response to the circumferential temperature variation. That is, the isolation characteristic of the conventional optical isolator device is degraded by the circumferential temperature variation.
In order to improve the isolation characteristic of the optical isolator device, an approach was proposed by Fukushima et al where two similar optical isolators were connected in cascade to realize an improved isolation. For details of the proposed optical isolator device, reference is made to a paper entitled "Improvement of Isolation Characteristic of Optical Isolator for Single Mode Optical Fiber" by Fukushima et al, published in Summaries of National Conference of The Institute of Electronics, Information and Communication Engineers on Light and Electromagnetic Wave held in 1984.
In the proposed isolator device comprising two similar optical isolators connected in cascade, the isolation characteristic is improved in comparison with a single optical isolator. However, the temperature dependency of the Faraday rotation angle of the Faraday rotator becomes twice effectively so that a desired isolation cannot be insured over a wide temperature range.